The invention relates to air bag systems for cars. FIG. 1 shows a conventional air bag system consisting of a number of firing stages 2, 4, a microcontroller 6 and a crash sensor 8. The firing stages 2, 4 switch ignition currents through squibs 10, 12 to fire the air bag at the appropriate time during a crash of the car. A high number of squibs requires a large number of wires, since two wires per squib are needed. Short circuits of these wires to ground or the battery voltage may cause unwanted firing of the airbags.
During a crash the electric connections to the car battery may be lost. Therefore all air bag systems contain an energy reserve capacitor (ERC)14 to keep the system operating for a limited time during the crash. This ERC capacitor 14 is charged to a relatively high voltage, for example 30 V, by means of a set up converter 16 which converts the 12 V battery voltage of the battery 18 to the high voltage. The high voltage is used to supply the firing stages 2, 4. A step down converter 20 converts the high voltage to a working voltage, usually 5 V, for the microcontroller 6 and the crash sensor 8.
FIG. 2 shows a conventional networked air bag system which uses a two-wire firing bus which connects multiple smart squibs 22, 24 to a bus driver 26. The two wires carry the data transmission signal and the power supply to the smart squibs 22, 24. The squibs 10, 12 are the igniters which actually fire the air bags. The energy for firing the squib is stored in a local capacitor inside the smart squibs 22, 24. The energy reserve for firing the squibs is now inside the smart squibs, but still energy reserve is needed for the crash sensor 8 and the microcontroller 6 which controls the bus driver 26.
One of the advantages of a networked air bag system is that a fault on one of the bus wires does not lead to unwanted deployment of a squib. If the whole firing bus is supplied from a floating power supply, the common mode voltage on the bus is undefined. This means that with either bus wire shortened to ground or to the battery voltage the system still operates correctly. To make a floating power supply a DC-DC converter 28 with a transformer 30 is needed. A transformer, however, is an expensive component.
It is an object of the invention to provide a transformerless air bag system. To this end the invention provides an air bag system as claimed in claim 1. The energy reserve capacitor is split into two parts: a main part to supply the microcontroller and the crash sensor, and a further part to supply the floating bus driver. The further part is connected to the main part by means of switches which open during a crash. The energy content of the further part is enough to supply the bus driver during the crash. The switches disconnect the further part of the energy reserve capacitor and all components connected to it from the rest of the system and thus a floating power supply is created during the crash.
The above and other objects and features of the present invention will become more apparent from the following detailed description considered in connection with the accompanying drawings in which:
FIG. 1 is a block diagram of a first conventional air bag system;
FIG. 2 is a block diagram of a second conventional air bag system; and
FIG. 3 is a block diagram of an air bag system according to the invention.